1. Field of Invention
The present invention relates to the use of nuclear response measurements of fuel flow and fuel quality combined with a process control system for optimization of air and coal stoichiometry at each burner in a coal-firing boiler.
2. Description of the Related Art
In the field of coal-firing boilers used especially by utility companies and industrial boiler operators, it is well known that increasingly stringent emissions limits continue to apply pressure to reduce NOx emissions from coal fired boilers. Years of investigation by utilities, boiler suppliers, and controls suppliers have determined that stoichiometries local to the burners must be maintained to achieve very low NOx emissions without negatively effecting combustion efficiency or boiler performance. Common barriers to lower NOx emissions include poor coal and air distribution which may also lead to high unburned carbon, high COx, boiler slagging, and oxygen and/or steam temperature imbalances.
To date, all low NOx firing systems are based on a pre-defined balance of air and coal at the burners. The air/fuel balance is maintained by adjusting the secondary air flow and, in some cases, the coal feed rate. Deviations from the design air/fuel balance at individual burners results in burners operating at a fuel lean or fuel rich condition. A fuel lean burner produces high NOx levels at elevated O2, resulting in a flue gas with high COx, high NOx, and increased LOI due to burners operating with poor stoichiometries. A fuel rich burner produces large amounts of COx, high LOI, and longer flames while lowering the oxygen level in the flue gas. Many coal fired boilers with poor air/fuel distribution experience problems such as: emission problems; increased unburned carbon in fly ash; distorted oxygen profile at the boiler outlet; uneven steam temperature profiles; flame impingement; increased slagging; and water wall heat waste.
In conventional coal handling systems that include adjustments to the coal feed rate as part of maintaining the air/fuel balance, the coal feed rate is based on either volume or weight. In the United States, gravimetric feeders are predominately used to supply coal to the pulverizer based on weight. More specifically, adjustments to the coal feed rate in conventional handling systems are designed to keep the coal flow rate constant for a specified demand.
The inventor of the present invention is a co-inventor of the subject matter disclosed in U.S. Pat. No. 7,006,919, titled “Real time continuous elemental measurement of bulk material,” issued on Feb. 28, 2006. In that patent, various methods and an apparatus for continuous real-time measurement of bulk material using gamma irradiation and neutron irradiation is disclosed. The '919 device includes a dual-energy gamma attenuation (DGA) device for monitoring bulk material flow and for producing a spectrum that is compared to a baseline spectrum to produce a relative weight/impurity ratio. A prompt gamma neutron activation analysis (PGNAA) device monitors the same bulk material flow and produces a spectrum that is compared to a library of spectrums to produce a relative component ratio. The relative component ratio is processed with the relative weight/impurity ratio to produce an absolute weight and impurity value, which is then processed with the relative component ratio to produce absolute component, or analyte, values.
The DGA analysis technique involves bombarding a bulk material with gamma rays from two gamma ray emitters of sufficiently different energies. The gamma rays interact with the bulk material resulting in the attenuation of the number of gamma rays transmitted through the bulk material. The gamma rays are typically detected by a scintillation crystal (typically NaI). The sum of these released gamma rays at these specific energies is referred to as an energy spectrum. The technology relies on the fact that elements with different atomic numbers attenuate gamma rays at specific energies in different ways. Thus, for low-energy gamma rays (i.e., those generated by a low energy gamma emitter such as Am-241), the attenuation of gamma rays is largely dependent on the atomic number of the atoms/elements present in the bulk material. For high-energy gamma rays (i.e., those generated by a high-energy gamma emitter such as Cs-137), attenuation is independent of the atoms/elements in the bulk material. Analysis of the energy spectrum leads to a determination of the bulk elemental composition of the bulk material.
DGA based sensors are known in the art. DGA devices are based on the premise that analyzed material will attenuate different energy gamma rays in fixed repeatable ways. A DGA device consists of a gamma energy source arrangement consisting of dual energy gamma emitters. The gamma emitters are chosen in such a way that the material to be analyzed will attenuate the different energy gamma rays in ways that are conducive to measuring one or more specific properties of the material being measured. One such application of DGA technology uses gamma ray sources to interrogate coal, with the assumption that the material of which the coal is composed will attenuate the differing energy gamma rays to produce a measurement that is conducive to determining coal ash content and density.
The multi-energy gamma attenuation (MGA) analysis technique involves the use of several gamma emitters at various energies to determine the bulk material quality and content. The apparatus used in such MGA analysis is a multiple-energy (three or more sources) gamma attenuation analyzer including a shielded source enclosure, a detector assembly, and a structural support framework defining an analysis zone in which the bulk material to be analyzed passes. The apparatus includes an MGA device to determine the absolute material density and content, and a computing/processing system for combining the resultant sensor data into quantities representative of the material quality. The MGA technique has a distinct advantage over the DGA technique in that the atomic/elemental interaction with the gamma energy takes place at several energies that depend on the atomic number of the atom/element encountered. Therefore, knowing the relative attenuation of gamma rays at the energies of interest and the mathematical reduction of a measured energy spectra against the known relative attenuations results in a determination of the quality and content of the bulk material. MGA analysis technique is described in detail in U.S. patent application Ser. No. 10/875,907, filed by Osucha, Swindell and Lee, and published on Dec. 20, 2004 as U.S. Patent Pub. No. 2004/0262524.
The PGNAA technique involves bombarding a bulk material sample with neutrons from a neutron emitter (typically Cf-252). The neutrons collide with atoms/elements in the sample, emitter housing, and/or an external moderator and are captured by the nuclei of atoms/elements present in the sample. The capture process often involves the release of gamma rays at energies specific to the captured atom/element. These gamma rays are detected typically by a scintillation crystal (typically NaI). The sum of the detected gamma energy at these specific energies is an energy spectrum. Analysis of the energy spectrum provides analytical information on the proportion of the various elements present in the bulk material.
As discussed in the '919 patent, various PGNAA based sensor systems are known. One such analyzer is that described in U.S. Pat. No. 4,582,992, titled “Self-Contained, On-Line, Real-time Bulk Material Analyzer,” issued to Atwell, et al., on Apr. 15, 1986, which uses PGNAA technology in an attempt to determine the elemental content of the bulk material. The described analyzer uses an arrangement of neutron sources and gamma ray detectors in an enclosed assembly to perform its analysis. A similar device, described in U.S. Pat. No. 6,362,477, titled “Bulk Material Analyser for On-Conveyor Belt Analysis,” issued to Sowerby, et al., on Mar. 26, 2002, uses PGNAA technology in a bulk material on-conveyor belt arrangement to analyze bulk material. Again, this analyzer uses a neutron source and gamma ray detectors in an enclosed assembly to perform its analysis.